In the oil and gas industry, sub-surface hydrocarbon-bearing formations are accessed by drilling from surface using a drill bit mounted on the end of a drill string.
The drill string typically comprises a bottom hole assembly (BHA) which may incorporate the drill bit, heavy walled drill collars, measurement while drilling (MWD) tools, logging while drilling (LWD) tools and rotary steerable drilling systems (RSDS). The BHA will be connected to a large number of relatively small diameter drill pipe sections which extend to surface.
Drilling BHAs will sometimes include check or float valves which allow drilling fluid to be pumped downhole through the drill string to the drill bit, but which prevent fluid from flowing into the drill string and up the string to surface. Indeed, some operators now insist that two independent check valves are provided in every drilling BHA, particularly when drilling high pressure high temperature (HPHT) wells. In the absence of one or more check valves in the BHA, in the event of a “kick” (a sudden increase in fluid pressure) or a sudden influx of gas from a hydrocarbon-bearing formation, the gas can percolate up and will then tend to expand and flow rapidly up through the bore, and up through the drill string; this is extremely dangerous. The blowout preventer (BOP) provided at the surface of the bore may be used to prevent fluid escaping from the annulus between the drill string and the wall of the hole. However, the BOP can only prevent fluid passing up through the drill pipe by severing the pipe with the blind-shear rams, which is of course a last resort method.
There are some disadvantages associated with including check valves in the BHA. Primarily, incorporating one or more check valves in the BHA prevents the drill string from self-filling as the drill string is made up and tripped or lowered into the bore in preparation for a drilling operation. To avoid the drill string collapsing due to the hydrostatic pressure exerted on the string by the fluid in the annulus, the operator has to “top fill” the drill string at intervals as the drill string is tripped into the bore.
It is known to provide flapper type check valves having an orifice in the flapper. This minimises the need for top filling, but the valve does not provide a complete pressure barrier and relying on such a valve may be unacceptable to some operators, particularly in HPHT wells. Alternatively, flapper type valves are available in which a sprung latch initially holds the flapper partially open, allowing self-filling. However, as soon as any fluid is pumped through the string, for example, a shallow test of the flow activated tools in the BHA, or to ensure that the jetting nozzles and the drill bit are not blocked, the flapper will open, releasing the latch, such that the flapper closes when the pumps are turned off. The drill string must then be top filled for the remainder of the tripping operation.
WO 2008/005289 describes a downhole well control device that includes a bypass one-way valve. However, as best understood, the one-way valve appears to be provided in combination with a bypass valve to permit flow from a pipe bore into an annulus when the bypass valve is open but to prevent flow from the annulus into the pipe bore.
Check valves or floats are used in non-drilling operations in combination with relatively large diameter bore-lining tubing such as casing and liner. Davis-Lynch, LLC offer Davis Self-Filling Float Shoes and Float Collars and Halliburton also offer an Advantage IPV Insert Poppet Valve, as described in U.S. Pat. No. 5,647,434. These valves or floats are run into a bore on the lower end of a string of bore lining casing or liner. Top filling a casing or liner string is relatively straightforward, due to the large diameter of the tubing. Also, an operator will often have a wash-down tool rigged up to allow circulation of fluid while running in hole, making it straightforward to top fill while running in. However, such check valves and floats are occasionally initially held open, for example by locating beads or balls between the valve member and valve seat, which balls or beads may be displaced by pumping fluid down through the string, allowing the valve to close. This obviates the requirement for top filling, however the primary reason for initially holding the valve open is to minimise the pressure surge created by running the large diameter tubing into the bore; the string is likely to be only slightly smaller than the bore, such that fluid would otherwise be trapped in the bore ahead of the string and the resulting pressure increase could damage or break down the formation, compromising subsequent cementing and production operations. However, the reduction in the risk of damaging the formation due to a pressure surge must be balanced against the risks involved in running an open large diameter string into the bore, such that some operators prefer to maintain the valves or floats closed, and run the tubing in relatively slowly. Also, if it is considered necessary or desirable to circulate fluid through the string including a valve held open by beads or balls, for example to facilitate progress of the string through a tight spot in the bore or to displace settled cuttings, an initially open valve will then close.
Once at the desired depth and following a cementing operation, the heavier liquid cement that has been pumped down through the string and the valve, and up into the annulus, will not be able to u-tube back into the casing/liner through the closed valve whilst the cement is setting.
U.S. Pat. No. 6,401,822 to Baugh describes a check valve assembly for the bottom of a casing string which is configured to provide a relatively high open area from a variety of flow paths to allow the string to be run in relatively quickly without fluid pressure build up in the formation. Multiple check valves are provided to provide for assurance of ultimate closure. The various flow paths and the check valves are closed by dropping a ball into the string when the casing has reached to desired depth.
Applicant's International Patent Application PCT/GB2012/052924, the disclosure of which is incorporated herein in its entirety, discloses a number of check valve configurations.